The present invention relates to a plant for transmitting electric power comprising a direct voltage network and at least one alternating voltage network connected thereto through a station. The station is adapted to perform transmission of electric power between the direct voltage network and the alternating voltage network and comprises at least one VSC converter adapted to convert direct voltage into alternating voltage and the converse, and an apparatus for controlling the converter. The plant further includes means adapted to calculate a pulse width modulation pattern, according to which the apparatus controls the converter for alternating voltage generation based upon a reference alternating voltage calculated through orders of reactive and active power flow given to the converter and the magnitude of the direct voltage of the direct voltage network at the station.
Such a plant has recently become known through the thesis "PWM and Control of Two and Three Level High Power Voltage Source Converters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm, 1995, in which publication such a plant for transmitting electric power through a direct voltage network for High Voltage Direct Current (HVDC) is described.
Before the issuance of the above thesis, plants for transmitting electric power through a direct voltage network for High Voltage Direct Current were based upon the use of line-commutated converters with thyristors or mercury-arc valves and CSC (Current Source Converter) converters. The development of IGBTs (Insulated Gate Bipolar Transistor, bipolar transistor having an insulated gate) for high voltage applications, and the suitability of connecting them in series in valves in converters, since they may easily be turned on and turned off simultaneously, has resulted in VSC (Voltage Source Converter) converters for forced commutation becoming an alternative. This type of transmission of electric power between a direct voltage network for High Voltage Direct Current being voltage stiff therethrough, and alternating voltage networks connected thereto, offers several important advantages with respect to the use of line-commutated CSCs in HVDC. In such systems, the consumption of active and reactive power may be controlled independently of each other, and there is no risk of commutation failures in the converter and thereby no risk of transmission of commutation failures between different HVDC links which may take place in line-commutation. Furthermore, it is possible to feed a weak alternating voltage network or a network without a generator of its own (a dead alternating voltage network).
In a plant of the type mentioned above, problems arise when the direct voltage on the direct voltage side of the station suddenly sinks or the alternating voltage on the alternating voltage side of the station suddenly increases, i.e., if the alternating voltage becomes too high with respect to the direct voltage for fulfilling the orders set. This may, for example, happen upon a suddenly increased tapping of active power of the alternating voltage network, and even if the plant in question has another station having an alternating voltage network connected thereto, and this station is adapted to try to keep the direct voltage of the direct voltage at a predetermined nominal value, the voltage regulation capability of this station will not be able to increase the corresponding power towards the direct voltage side, therefore, the direct voltage sinks. Thus, in such a case, the alternating voltage gets too high with respect to the direct voltage as a consequence of the sinking direct voltage. The problem so arising is associated with the means for calculating the pulse width modulation pattern, since so-called overmodulation occurs when a reference alternating voltage which is too high with respect to the direct voltage is reached. The consequence of this is that harmonics other than the characteristic ones are generated, and thereby disturbances are created in the networks and the equipment connected thereto.